DVDs (digital versatile disks) containing various contents such as movies have been widespread. In recent years, next-generation disks such as BDs (Blue-ray disks) and HD-DVDs (high-definition DVDs) containing various contents are becoming widespread in addition to the DVDs. Moreover, the use of a home theater system to reproduce the contents of the next-generation disks is spreading. The home theater system is, e.g., a 5.1-channel sound field system (hereinafter, the term “channel” is represented by “ch”) installed at home. Unlike the DVDs containing audio information provided for the 5.1ch sound field system, the next-generation disks contain uncompressed high-quality audio information provided mainly for a LPCM (linear pulse code modulation) 7.1ch sound field system. For the reproduction of such contents, when an AV amplifier connected to a reproduction apparatus such as a BD player is compatible with the 7.1ch sound field system, a speaker system compatible with the 7.1ch sound field system can be used to perform multi-channel reproduction with precise auditory localization and high sound quality.
FIG. 3 shows the configuration of a conventional audio signal reproduction system compatible with the 5.1ch sound field system.
In FIG. 3, an AV amplifier 110 includes the following: a communication portion 101 that can communicate with external equipment 106 via a cable 107; a control portion 102 that converts the information received by the communication portion 101 into 5.1ch audio information; digital-to-analog converters (referred to as “DAC” in the following) 103a to 103f that convert the audio information (digital signals) output from the control portion 102 into analog audio signals; amplifiers 104a to 104e that are connected to the DACs 103a to 103e, respectively; a central processing unit (CPU) 111 that controls the communication portion 101 and the control portion 102; a ROM (read only memory) 112; and a RAM (random access memory) 113.
The boxes indicated by a broken line in the control portion 102 show the audio information for each channel. In FIG. 3, “Lch” represents the audio information to be output from a left front speaker, “Rch” represents the audio information to be output from a right front speaker, “Cch” represents the audio information to be output from a center speaker, “SLch” represents the audio information to be output from a left rear speaker, “SRch” represents the audio information to be output from a right rear speaker, and “SWch” represents the audio information to be output from a sub-woofer.
The external equipment 106 (e.g., a BD player) and speakers 105a to 105f compatible with the 5.1ch sound field system are connected to the AV amplifier 110. The speaker 105f is connected to the AV amplifier 110 via an amplifier 104f. In this case, the speaker 105f may contain an amplifier and be connected directly to the AV amplifier 110.
The BD audio information reproduced and output from the external equipment 106 is input in the form of digital data to the communication portion 101 of the AV amplifier 110 via the cable 107. Then, the audio information received by the communication portion 101 is input to the control portion 102 and decoded to 5.1ch audio information. The audio information decoded by the control portion 102 is input to the individual DACs 103a to 103f for each channel and converted into analog audio signals. The audio signals output from the DACs 103a to 103f are amplified by the amplifiers 104a to 104f and output through the speakers 105a to 105f to the outside as sounds, respectively.
Patent Document 1 discloses down-mixing 7.1ch audio information so that it can be reproduced with the 5.1ch sound field system.    Patent Document 1: JP 2005-341208 A